1. Field of the Invention
The present invention generally relates to scanning line number converting devices for video signals and to down-converters and picture-in-picture TV receivers using the same. More particularly, the present invention relates to a scanning line number converting device which reduces the scanning line number of a received video signal, and to a down-converter which converts, using the scanning line number converting device, a high-resolution TV signal with a large number of scanning lines into a video signal of NTSC system, and a picture-in-picture television receiver which projects, also using the scanning line number converting device, a video signal with a reduced number of scanning lines onto an ordinary TV screen to display two pictures.
2. Description of the Background Art
A scanning line number converting device which properly processes an interlaced input television signal to output a television signal having a different number of scanning lines from that of the input signal has been generally incorporated in a variety of apparatuses embodying specific purposes. As such apparatuses having specific purposes, for example, a picture-in-picture TV receiver (see "Nikkei Electronics", Apr. 14, 1980, Japanese Patent Publication No. 59-37913, Japanese Patent Laying-Open No. 62-269482, and so on) and a high-resolution TV/NTSC down-converter (refer to, for example, Japanese Patent Application No. 1-120128) are known.
The above-mentioned apparatuses all comprise a scanning line number converting device. Such a scanning line number converting device is configured according to purposes of use of an apparatus in which the device is incorporated. The scanning line number converting device is generally configured, however, to make conversion such that the scanning line number of an output signal is smaller than that of an input signal.
First, prior art of such a scanning line number converting device will be described.
A scanning line is generally considered as a vertical direction sampling in sampling a TV picture plane bidimensionally. An operation for reducing the scanning line number, therefore, is equal to that of reducing the vertical direction sampling frequency of the TV picture plane. Such an operation can be implemented by functions of the following two components.
One is a band limiting filter which limits a vertical spatial frequency component of a TV picture plane to less than 1/2 of a reduced vertical direction sampling frequency. The other is a scanning line thinning-out circuit which thins out scanning lines to reduce the vertical direction sampling frequency of a TV picture plane. The functions of those components are in accordance with the sampling theorem and their operation principles are self-evident.
FIG. 1A is a diagram showing the scanning line position of an interlace signal for each field f1, f2. In the diagram, "o" indicates a scanning line, position of which is shifted in the vertical direction by one line from one field to another. FIG. 1B is a diagram showing the scanning line position of a non-interlace signal for each field. In the diagram, "o" and "x" indicate together scanning lines. However, "o" is one corresponding to the interlace signal, while "x" is one properly produced by interpolating the interlace signal. Further, the scanning lines are located in the same position for all fields.
In FIGS. 1A and 1B, the abscissa represents time base with a field cycle as a unit and the ordinate represents the vertical direction with a scanning line interval as a unit.
FIG. 2 is a diagram showing a basic structure of a device which receives such an interlace signal to convert its scanning line number. FIG. 3 is a diagram for explaining operation of the scanning line number converting device shown in FIG. 2.
A signal applied to an input terminal 500 is limited in band by a vertical spatial frequency limiting low-pass filter 501 and then supplied to a thinning-out circuit 502. In the thinning-out circuit 502, the scanning lines are thinned out as shown in FIG. 3 so that scanning line signals corresponding to the signs "x" are outputted from an output terminal 503.
In FIG. 3, "o" indicates a scanning line corresponding to an input signal and "x" indicates one corresponding to an output signal. Further, " " indicates a tap range of the vertical spatial frequency limiting low-pass filter.
A TV signal entered in such a scanning line number converting device is an interlaced signal. Therefore, the output signal shown in FIG. 3 which has a reduced number of scanning lines has been also interlaced.
Meanwhile, an apparatus comprising a scanning line number converting device which outputs such a signal further processes the signal according to its purposes of use.
In such a case, since the signal to be handled has been interlaced, it is often required to make determination as to interlace sequence using field determining means. In the prior art, therefore, the apparatus has been configured such that field determination is made for the interlaced signal to execute the following signal processings based on the determination results.
In the following, reasons why field determination is required for signal processings will be described in connection with the above-mentioned two apparatuses.
First, description will be made on a picture-in-picture TV.
The basic structure of a picture-in-picture TV is described in "Nikkei Electronics" Apr. 14, 1980. That is, the picture-in-picture TV comprises a picture memory for absorbing time difference between a video signal for a main picture and that for a sub-picture and is configured such that the video signal for the sub-picture is written in the picture memory in synchronization with a synchronizing signal contained in itself and read out in synchronization with that contained in the video signal for the main picture to display the sub-picture in a predetermined position in the main picture.
The picture-in-picture TV configured in such a manner involves two problems in terms of technique. In prior art, field determining means has been employed to solve those problems.
Both of the two problems are generally caused by disagreement of signal phase between the video signal for the main picture and that for the sub-picture.
First, when there is no correspondence of interlace relationship between the video signal for the main picture and that for the sub-picture, since the above-mentioned picture memory is generally controlled on a field-by-field basis, the interlace relationship of the displayed sub-picture may be undesirably inverted (problem of incomplete interlace).
If the interlace relationship is inverted in such a manner, intense line flickers, double-image disturbance and the like occur on the sub-picture.
Secondly, when the vertical synchronizing signal phases of the video signal for the main picture and that for the sub-picture do not meet a certain relationship, the video signal for the sub-picture being read out of the picture memory is rewritten by another information for the subsequent field, so that pictures of different fields may be displayed on the upper and lower sides of the boundary between the main picture and the sub-picture (boundary problem).
When pictures of different fields are displayed on the upper and lower sides of the boundary between the main picture and the sub-picture, scanning lines on the boundary are clearly observed especially for moving pictures, causing visual disturbances. Further, since the interlace relationship is inverted between the upper and lower sides of the boundary, not only the scanning lines on the boundary are observed, but at the same time, the above-mentioned first problem occurs. That is, a normal picture can be displayed only on either of the upper and lower sides of the boundary, and line-flickers, double-image disturbance and the like will occur on the remaining side.
These two problems are basic ones which must be solved to enhance picture quality of the picture-in-picture television and for which a method using field determination means has been proposed.
For the first problem, field determination is first made on both video signals of the main and sub-pictures. The video signal for the sub-picture is written in a predetermined area of the picture memory based on the result of the field determination for itself. Further, the video signal for the sub-picture is read out with an appropriate starting phase, based on the result of the field determination for the main picture video signal. Thus, correspondence of the interlace relationships between the video signal for the main picture and that for the sub-picture is achieved, as has been proposed in Japanese Patent Publication No. 59-37913.
For the second problem, the picture memory is divided into four areas, two areas of which are allotted to a first field and the other two ones to a second field. Further, a passing prevention circuit is provided to make control such that reading and writing are not simultaneously applied to the same area. Thus, the so-called "passing" where the video signal for the sub-picture being read out of the picture memory is rewritten by another information of the subsequent field can be prevented, as has been proposed in Japanese Patent Publication No. 62-269482.
More specifically, the video signal for the sub-picture is written in a predetermined area of the picture memory, based on the result of the field determination for itself. Further, the passing prevention circuit makes field determination as to the video signal of the main picture and then reads out the sub-picture video signal from a first-written one out of the two areas which have field information corresponding to the determination result. Thus, reading and writing are applied to each field of the picture memory in a first-in-first-out fashion, allowing reading of field information to precede writing of another field information, so that "passing" can be prevented as described above.
The first and second problems can be, therefore, solved individually using the field determination means. Furthermore, if the control function described as a solution to the first problem which can bring about correspondence of the interlace relationships between the video signal for the main picture and that for the sub-picture is added to the passing prevention circuit described as a solution to the second problem, the two problems can be solved at a time.
Therefore, it is apparent that the field determination means is indispensable for the picture-in-picture TV technology.
Subsequently, description will be made on a high-resolution/NTSC down-converter.
A high-resolution TV/NTSC down-converter converts an interlaced high-resolution TV signal of a field rate of 60.00 Hz having 1125 scanning lines per one frame into an interlaced television signal of a field rate of 59.94 Hz having 525 scanning lines per one frame.
The converter has, therefore, two points to be noted; one is frame rate conversion and the other is scanning line number conversion.
Among them, problems involved in the frame rate conversion can be considered as the same ones accompanying the phase correspondence between the main and the sub-pictures as have been described in connection with the picture-in-picture TV. Therefore, the boundary problem due to a passing may be expected to occur.
Those problems can be, however, solved with the application of the field determination means and by use of the method described in Japanese Patent Laying-Open No. 62-269482.
However, many of the currently available down-converters do not carry out the conversion of frame rate. Therefore, while it has been pointed out that the boundary problem will arise in a future frame rate conversion, problems to be solved themselves have not yet come up in reality.
As to the scanning line number conversion, specific examples are described in the previously mentioned Japanese Patent Application No. 1-120128 and the like.
Now, description will be made on the high-resolution TV/NTSC down-converter. The high-resolution TV/NTSC down-converter comprises field determination means for making determination on an interlaced input signal as to whether the present field is even one or odd, scanning line number converting means for making an odd field and an even field of the interlaced video signal each contain 525 scanning lines and registration means for positioning scanning line signals of either an odd field or an even field outputted from the converting means in registration with those of the other, and converts the interlaced input signal into a non-interlaced TV signal having 525 scanning lines per one frame.
Since the signal outputted from the scanning line number converting means has been interlaced, the position of the scanning lines differs from one field to another. Therefore, the down converter is adapted to position, based on the results of the field determination, scanning lines of either field in registration with those of the other, preventing line flickers taking place.
The field determination means is, therefore, one of the indispensable elements to prevent occurrence of the line flickers.
As has been described with reference to the two apparatuses, a conventional apparatus comprising a scanning line number converting device employs the field determination means as an indispensable element for achieving its purposes.
When an apparatus is configured as described above and allows proper operation of the field determination means, both of the scanning line number converting device and the apparatus itself can properly operate, obviating any possible problems.
When a reproduced video signal from, for example, a home VTR is applied, however, the apparatus may not operate in order.
This is because the field determination means used as a solution to the problems may possibly malfunction against the video signal reproduced by the home VTR.
Such a malfunction of the field determination means in use with the video signal reproduced by the home VTR is attributable to noise which has been mixed in the vicinity of the vertical synchronizing signal due to switching of heads. Since the field determination means makes determination as to field sequence generally by comparing in phase the horizontal synchronizing signal and the vertical synchronizing signal, it may make an erroneous field determination when such noise has mixed in the vicinity of the vertical synchronizing signal as described above. Such a malfunction occurs with high probability in the case of particular reproductions such as picture search or slow reproduction.
Further, such a malfunction will take place not only for the video signal reproduced by the home VTR, but for those signals from a static picture photoplayer or a TV game machine. Malfunctions caused by such video signals are not, however, due to the noise as described above, but due to the output video signals themselves that have not been originally interlaced.
Operation of the field determination means for such non-interlaced video signals can not be generally defined at all. For example, it is entirely uncertain whether determination outputs of either one of the first and second fields should continue to be outputted or determination outputs of the first and second fields should be irregularly outputted. If the above-described solutions to the first and second problems are to be applied to such outputs, there is only a 50-50 chance that the problems can be effectively solved. In other words, there exists a possibility that the problems may not be effectively solved.
Thus, when the malfunction of the field determination means is taken into consideration, such signal processings as depending on the field determination means have limits.
Summarizing all described above, the following consequences can be obtained. That is, in a conventional scanning line number converting device and an apparatus comprising such a device, the field determination means has been used an an indispensable element for achieving their purposes of use. However, the field determination means may perform a malfunction and the resulting erroneous determination will hinder achievement of the whole purposes of the apparatus comprising the scanning line number converting device, as can be seen in the occurrence of line flickers, for example.
Such problems can be avoided by making the scanning line number converting device output non-interlace signals. As to a non-interlace signal, there is no such notion as "field". Therefore, by employing the non-interlace signal, no field determination means is required even in an apparatus comprising a scanning line number converting device.